CompuServe Thread

Doug, Llanoite (sometimes Llanite) is a rhyolite that contains bright pink orthoclase – sometimes _nice_ crystals, with knee twins. The blue is indeed quartz. It's really blue, too, not just bluish. The quartz contains reduced chromium, hence the blue color. And yes, Llanite does come from dikes extruded from the Llano batholith. There are a lot of nice igneous species around Llano, including the blue topaz you mentioned. There is a stream that washes down off the south face of the Enchanted Rock exfoliation zone that has placer deposits of topaz and other dense minerals, including some zircon sand, if you dig deep enough in the stream bed. I've collected a couple of ~1cm topaz in an afternoon with a sieve, three shovels, and two highly energetic kids. The blue quartz dikes are separate from the llanoite rhyolite. In some of the exfoliation domes that are exposed, there are largish (40 cm wide) dikes of pale blue quartz (not the Llanite blue) that go on for hundreds of meters. They're typically not very pretty, since every tourist on earth seems to think that it should be easy to hammer out a piece. Just to show you how rude some people can be (and perhaps why mine owners have become so anti-rockhound), the city of Llano put up a nice rest stop right outside of one of the quarries, and the quarry granted access to part of their tailings pile. It's a -GREAT- place to collect, I walked out with a biotite flake almost 40cm across. In the rest stop itself, the quarry provided nice granite picnic tables – nothing special, rockwise, just nice granite. They also placed a set of fossilized turbidites that were hauled out of the man-made lake (Buchannen) while the dam was being built. So called "tourists" hammered off the corners of the tables, and smashed one of the turbidite boulders. The tables were plain vandalism, destroying the fossils was ghoulish. — Norm